Method and extrusion head for extruding a polymeric material

ABSTRACT

An extrusion method and head for depositing by extrusion a polymeric material intended, in particular, for the production of cables includes a core and a coating layer arranged at a radially outer position with respect of the core. The method includes the steps of feeding the polymeric material in a feeding duct of the extrusion head and of conveying a part of the polymeric material towards an undercut extending in the extrusion head in proximity of an end of the feeding duct. The method provides a sealing force between a head body and a conveyor element of the extrusion head exerted by means of the pushing action exerted by the polymeric material in the undercut in a direction substantially perpendicular to the longitudinal axis of the head body.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/IT01/00653, filed Dec. 21, 2001, the content of which isincorporated herein by reference.

FIELD OF THE INVENTION

In a first aspect thereof, the present invention concerns a method forextruding a polymeric material by means of an extrusion head.

The present invention also concerns an extrusion head for extruding apolymeric material as well as a conveyor element and a head body makingpart of said extrusion head.

Moreover, the present invention concerns an extrusion apparatuscomprising said extrusion head.

For illustrative purposes, a possible field of application of thepresent invention is that of the production of cables including at leastone transmissive element and at least one coating layer arranged at aradially outer position with respect of said transmissive element. Moreprecisely, the present invention may advantageously be applied tounipolar or multipolar cables of the electric type for transporting ordistributing energy, or of the optical type or of the mixedenergy/telecommunications type.

PRIOR ART

A cable generally comprises at least one transmissive element(electrical, optical or electro-optical) and at least one coating layermade of polymeric material arranged at a radially outer position withrespect of said transmissive element.

The polymeric material of said coating layer is selected so as toprovide the latter with semiconductive properties, electrical insulationor mechanical protection properties depending on the type of cable underconsideration and of the position of said layer within said cable.

In order to manufacture a coating layer made of polymeric material, theprior art provides for the use of an extrusion head comprising:

-   -   a) a head body provided with an inner cavity coaxially extending        with a longitudinal axis of the head body; and    -   b) at least one conveyor element mounted in the inner cavity of        the head body about said longitudinal axis, the conveyor element        including:    -   b1) a tubular body provided with an inner cavity coaxially        extending with the longitudinal axis of the head body; and    -   b2) at least one distribution channel formed on the outer        surface of the tubular body and defining an upstream portion and        a downstream portion of the tubular body.

In greater detail, during the production process of a cable, the innercavity of the conveyor element is intended for receiving thesemi-finished cable produced up to this point and upon which a coatinglayer of polymeric material is to be deposited by extrusion, whereas thedistribution channel is intended for distributing said polymericmaterial on the outer surface of the conveyor element arrangeddownstream of the distribution channel.

The extrusion heads of the prior art comprise, moreover, a feeding ductof the polymeric material formed in the head body and a lamination ductdefined between the head body and the conveyor element.

The conveyor element, therefore, carries out the function ofcircumferentially distributing in a uniform manner the polymericmaterial intended for forming the coating layer of the cable into thelamination channel.

However, in the extrusion heads of the prior art the Applicant has notedan inadequate seal between the conveyor element and the head bodyupstream of the distribution channel, with consequent possible leakagesof the polymeric material in said zone of the extrusion head.

Regarding this, it can be emphasized that any possible leakages of thepolymeric material towards the zone of the extrusion head arrangedupstream of the distribution channel cause stagnations of the polymericmaterial which, also due to the effect of the temperature (for exampledue to a partial cross-linking), may cause the formation of clots ofsaid polymeric material.

Once formed, said clots may detach from the leakage zone and may flow inthe inlet mixture flow moving towards the lamination channel.

The presence of said clots, which constitute a dishomogeneity in thecoating layer of the cable being produced, not only negatively affectsthe aesthetic effect of the cable but, depending on the type of coatingbeing deposited, may also negatively affect the properties of the cable.

For example, if the coating layer being produced is the electricalinsulation layer, the occurring of any leakages during the deposition ofthe latter and the formation of clots in the electrical insulation layermay cause a deterioration of the electrical insulation properties of thecable, said drawback being particularly undesired above all in the caseof cables for transporting or distributing high and extra high voltageelectrical energy.

In the present description, the term “low voltage” indicates a voltagelower than about 1 kV, the term “medium voltage” indicates a voltage ofbetween about 1 kV and about 30 kV, the term “high voltage” indicates avoltage of between about 30 kV and about 220 kV, whereas the term “extrahigh voltage” indicates a voltage greater than about 220 kV.

In order to avoid or at least reduce said leakages, some technicalsolutions are known which are only partly suitable for the purpose andwhich, in general, involve a plurality of drawbacks essentiallyconnected to an increased complexity of mounting the conveyor in thehead body and/or to problems of stagnation of the polymeric material.

For example, extrusion heads comprising a head body the radially innersurface of which is provided with a profile of the substantially conicaltype are known. Said structure involves the need to operate withparticularly close constructive tolerances during the production stepsand, at the same time, involves the need to operate with the maximumprecision also during the assembly steps.

Moreover, said embodiment is particularly effective above all with flowsof polymeric material fed at a low pressure and causes an increase inthe assembly complexity, in the assembly time and in the productioncosts.

Extrusion heads are also known which provide for the use of a sealinggasket (for example an O-ring) in the coupling zone between the headbody and the conveyor.

However, said solution determines the formation of undesired stagnationzones of the polymeric material with the consequent need to makefrequent interventions to clean the extrusion head, as well as toreplace the sealing gasket.

SUMMARY OF THE INVENTION

In order to overcome the drawbacks of the prior art described above, theApplicant has perceived the need to provide a method for depositing byextrusion a coating layer of polymeric material by means of an extrusionhead capable of avoiding any undesired leakages of said material, whileensuring simplicity and rapidity of assembly.

The Applicant has found that it is possible to accomplish an effectiveseal between the conveyor element and the head body of an extrusion headby exploiting the pushing action which is exerted by the polymericmaterial fed in the corresponding feeding duct.

The Applicant has found that it is possible to exploit the pressure ofthe polymeric material entering an extrusion head by shaping the latterin a suitable manner in proximity of the axially inner end of thefeeding duct of the polymeric material obtained in said extrusion head.

Therefore, in accordance with a first aspect thereof, the presentinvention refers to a method for depositing by extrusion a coating layerof polymeric material by means of an extrusion head comprising a headbody and at least one conveyor element coaxially mounted with said headbody about a common axis, said head body comprising at least one feedingduct for feeding said polymeric material in a lamination channel definedbetween said head body and said conveyor element, said method comprisingthe steps of:

-   -   a) feeding said polymeric material in said feeding duct;    -   b) conveying a part of said polymeric material towards at least        one undercut arranged in proximity of an end of said feeding        duct, said part of said polymeric material exerting a sealing        action between said head body and said at least one conveyor        element.

In accordance with the present invention, said sealing action is carriedout by said part of said polymeric material by a pushing action exertedin a direction substantially perpendicular to said common axis.

Thanks to the fact that the aforementioned undercut is formed inproximity of an end of the feeding duct at the interface between thehead body and the conveyor element, i.e. where there is the greatestpressure of the flow of the polymeric material entering the extrusionhead, the method of the invention advantageously allows to obtain aneffective sealing action against any possible leakages of the polymericmaterial.

Moreover, the method according to the present invention canadvantageously be carried out without modifying the conventionalgeometry of the mounting surfaces of the head body and of the conveyorelement. In such a way, it is possible to ensure a constructivesimplicity both of the conveyor element and of the head body, as well asan assembly simplicity of the conveyor element in the head body.

In accordance with a first preferred embodiment of the method accordingto the invention, the aforementioned undercut is formed in at least onedistribution channel (also called compensation channel) formed on theouter surface of the conveyor element and the sealing action of saidmethod is exerted by the entering polymeric material which exerts apushing action of the conveyor element against the head body thusavoiding, or at least minimising, the risk of leakages between the headbody and the conveyor element.

Said operating method can be carried out also due to the fact that thepolymeric material is generally fed under pressure, said pressure beingimparted by the extrusion screw arranged upstream of the feeding duct ofthe polymeric material and depending also on the physical-chemicalproperties (for example the viscosity) of the polymeric material. Itcan, indeed, be emphasized that the sealing action according to themethod of the present invention is particularly effective above all ifthe entering polymeric material is fed at high pressure so that saidmaterial can exert said pushing action. On the contrary, if the enteringpolymeric material is fed at low pressure or is in a transient condition(for example at the beginning or at the end of the feeding step of thepolymeric material to the extrusion head), the aforementioned sealingaction is advantageously and sufficiently ensured by the interferencebetween the head body and the conveyor element.

In accordance with said first preferred embodiment, said sealing actionis accomplished by pushing an elastically deformable, substantiallylip-shaped, sealing element which is integrally formed with saidconveyor element and which extends in a direction substantially parallelto said common axis. In accordance with said first preferred embodiment,an elastically deformable lip defined between the aforementionedundercut and the radially outer surface of the conveyor element isformed in the conveyor element. Therefore, the seal between the headbody and the conveyor element is accomplished thanks to the pressureexerted by the polymeric material against said lip, said pressureelastically deforming the lip in the direction of the head body and thusattaining the desired sealing action.

In accordance with a second preferred embodiment of the method of theinvention, the aforementioned undercut is formed in the feeding duct ofthe head body and the aforementioned sealing action is carried out bythe entering polymeric material which exerts a pushing action of thehead body against the conveyor element nullifying, or at leastsubstantially reducing, the risk of leakages between the conveyorelement and the head body.

In accordance with said second preferred embodiment of the methodaccording to the invention, the sealing action is carried out by theentering polymeric material which exerts a pushing action upon anelastically deformable, substantially lip-shaped, sealing element whichis integrally formed with the head body and which extends in a directionsubstantially parallel to said common axis. In accordance with saidsecond embodiment, said lip is advantageously capable of performing theseal between the head body and the conveyor element when the lip ismoved by the pushing action exerted against the latter by the enteringpolymeric material.

In a second aspect thereof, the present invention refers to an extrusionhead for extruding a polymeric material comprising:

-   -   a) a head body provided with an inner cavity coaxially extending        with a longitudinal axis of said head body; and    -   b) at least one conveyor element mounted within said inner        cavity of the head body about said longitudinal axis, said        conveyor element including:    -   b1) a tubular body provided with an inner cavity coaxially        extending with said longitudinal axis;    -   b2) at least one distribution channel formed on the outer        surface of said tubular body and defining an upstream portion        and a downstream portion of said tubular body;    -   characterized in that said distribution channel is provided with        at least one undercut axially extending in said upstream portion        of said tubular body.

Preferably, said at least one undercut axially extends in said upstreamportion of said tubular body along a direction substantially parallel tosaid longitudinal axis.

Advantageously, said first embodiment of the extrusion head of theinvention accomplishes the seal between the head body and the conveyorelement thanks to the particular configuration of the distributionchannel and to the pushing action exerted by the polymeric material.

Preferably, the aforementioned undercut has a length (in the axialdirection) of between about 3 mm and about 30 mm, preferably of betweenabout 4 mm and about 12 mm.

In such a way, therefore, a seal is ensured at pressures of thepolymeric material comprised between about 0 bar and about 1500 bar.

In accordance with a preferred embodiment, the undercut is substantiallyU-shaped.

Thanks to said a configuration of the undercut, the risk of formation ofstagnation zones of the material is advantageously further reduced.

Preferably, the bottom wall of the undercut is substantiallyperpendicular to the longitudinal axis of the head body.

Preferably, the bottom wall of the undercut is connected to the bottomwall of the distribution channel by means of a curvilinear part.Preferably, said curvilinear connection part has a bending radiuscomprised between about 0.5 mm and about 15 mm.

Advantageously, the presence of said curvilinear connection part furtherallows to avoid the formation of stagnation zones of the polymericmaterial entering the feeding duct.

In accordance with a preferred embodiment of the invention, the undercutdefines an elastically deformable sealing element, substantiallylip-shaped, positioned in the upstream portion of the tubular body ofthe conveyor.

According to the aforementioned embodiment, said lip is advantageouslycapable of accomplishing the seal between the head body and the conveyorelement when the lip is pushed against the head body by the pushingaction of the polymeric material.

Preferably, the bottom wall of the undercut is connected to the lip bymeans of a curvilinear part. Preferably said curvilinear connection parthas a bending radius comprised between about 0.5 mm and about 15 mm.

Preferably, the curvilinear connection parts between the bottom wall ofthe under cut and the lip and between the bottom wall of the undercutand the bottom wall of the distribution channel have the same bendingradius.

Generally, the conveyor comprises at least one pair of distributionchannels which begin at the feeding duct formed in the head body andrespectively extend on opposite sides along the outer surface of theconveyor so as to concern the whole circumferential development of saidconveyor and to distribute the polymeric material, entering from saidfeeding duct, onto the outer surface of the conveyor (also known aslamination zone).

Preferably, each distribution channel of the conveyor has asubstantially helical development.

Preferably, each distribution channel is tapered, i.e. the length (inthe axial direction) of the bottom wall of said channel decreases alongthe development thereof in the circumferential direction. Moreprecisely, the length of said bottom wall (without considering thelength of the aforementioned undercut) is greater at the feeding ductand gradually decreases as the distribution channel heads into thelamination channel.

Moreover, preferably also the depth (measured in the radial direction)of the distribution channel decreases passing from the portion of saiddistribution channel arranged in proximity of the end of the feedingduct of the elastomeric material towards the portion of saiddistribution channel arranged in proximity of the lamination channel.

Said constructive solutions allow the maximum flow rate of the enteringpolymeric material to be received at the portions of the distributionchannels with a greater length (in the axial direction) and with agreater depth (in the radial direction), and thus with a greater volumecapacity.

Moreover, preferably the aforementioned undercut has a length (measuredin the axial direction) progressively decreasing along thecircumferential development of the distribution channel. More precisely,advantageously said undercut, and consequently the aforementionedelastically deformable lip defined by the same, has a maximum lengthwhere there is the greatest sealing action, i.e. at the zone of theextrusion head where there is the greatest pressure exerted by theentering polymeric material, i.e. at the feeding duct of the head body.

Preferably, the aforementioned undercut extends only along a portion ofpredetermined length of the distribution channel. In other words,preferably said undercut is no longer present (i.e. the undercut has alength equal to zero) before the distribution channel heads into thelamination channel.

Preferably, the undercut of the feeding duct has a length equal to zero(i.e. there is no longer the undercut) at the end of the distributionchannel in fluid communication with the lamination channel.

Preferably, the head body and the conveyor element are mounted withinterference by carrying out one or more heating steps alternated withone or more cooling steps in order to allow the insertion of theconveyor element within the head body.

Said type of assembly ensures a suitable sealing action when theelastomeric material is fed at low pressures (for example in a transientcondition), condition in which the lip remains substantially undeformedsince the pushing action of the polymeric material is lacking.

Preferably, the extrusion head according to the present invention canadvantageously be used in the production processes of cables fortransporting or distributing high or extra high voltage energy, inparticular for depositing by extrusion coating layers, such as theelectrical insulation layer or the inner or outer semiconductive layersof the cable, which require the use of particularly viscous polymericmaterials which have to be fed at high pressures, for example comprisedbetween 0 bar and 1500 bar, preferably between 100 bar and 600 bar.

The extrusion head further comprises a lamination channel definedbetween the conveyor element and the head body downstream of thedistribution channel, the lamination channel being in fluidcommunication with the distribution channel and with an outlet openingformed in the extrusion head.

The extrusion head further comprises a male die and a female die mountedin the head body about the longitudinal axis thereof, downstream of theconveyor element.

In a further embodiment of the invention, the extrusion head furthercomprises at least one intermediate die mounted in the head body betweenthe male die and the female die.

In accordance with a second embodiment, the present invention concernsan extrusion head for extruding a polymeric material comprising:

-   -   a) a head body including:    -   a1) an inner cavity coaxially extending with a longitudinal axis        of said head body;    -   a2) a feeding duct for feeding said polymeric material in said        head body; and    -   a3) at least one distribution channel formed on the inner        surface of said head body and in fluid communication with said        feeding duct, said distribution channel defining an upstream        portion and a downstream portion of said head body, and    -   b) at least one conveyor element mounted in said inner cavity of        said head body about said longitudinal axis, said conveyor        element comprising a tubular body provided with an inner cavity        coaxially extending with said longitudinal axis, characterized        in that said distribution channel is provided with at least one        undercut axially extending in said upstream portion of said head        body.

Preferably, said at least one undercut axially extends in said upstreamportion of said head body along a direction substantially parallel tosaid longitudinal axis.

In accordance with said second embodiment of the extrusion headaccording to the invention, substantially the same advantages mentionedabove with reference to the first embodiment of the extrusion head areachieved.

Nevertheless, in accordance with said second embodiment, the sealingaction between the head body and the conveyor element is exerted by thepolymeric material on the side of the head body instead of on the sideof the conveyor element as in the case of the aforementioned firstembodiment.

In accordance with said second embodiment, the aforementioned undercutof the feeding duct of the head body has a length (measured in the axialdirection) comprised between about 3 mm and about 30 mm, preferablybetween about 4 mm and about 12 mm.

Preferably, said undercut is substantially U-shaped. Said particularconfiguration allows, in fact, to further minimize the risk of formationof stagnation zones of the polymeric material.

Preferably, the bottom wall (in the axial direction) of said undercut issubstantially perpendicular to the aforementioned longitudinal axis ofthe head body.

Preferably, the aforementioned undercut defines an elasticallydeformable, substantially lip-shaped, sealing element in the upstreamportion of the head body.

Preferably, the bottom wall of said undercut is connected to theaforementioned lip by means of a curvilinear part.

Preferably, said curvilinear part has a bending radius comprised betweenabout 0.5 mm and about 1.5 mm.

In accordance with a preferred embodiment, the distribution channelformed in the head body has a development of substantially helical type.

Preferably, said distribution channel is tapered.

Preferably, the undercut of said distribution channel has a decreasinglength along the development of the distribution channel. In such a way,the elastically deformable lip defined by the undercut has a greaterlength and, thanks to this, a greater effectiveness of seal, exactlywhere the sealing action is most needed, i.e. in correspondence of thefeeding duct.

Moreover, according to two further aspects, the present invention refersto a conveyor element and to a head body for an extrusion head forextruding a polymeric material having the features defined,respectively, in claims 22 to 32 and in claims 33 to 39.

Furthermore, the present invention refers to an apparatus for extrudinga polymeric material comprising an extrusion head such as the headdefined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will becomeclearer from the description of two embodiments of a method fordepositing by extrusion a polymeric material according to the invention,made hereafter with reference to the attached drawings in which, forillustrative and not limiting purposes, two extrusion heads for carryingout said method are represented. In the drawings:

a) FIG. 1 is a view in longitudinal section of a first preferredembodiment of the extrusion head according to the invention.

b) FIG. 2 is an enlarged view of a detail of the extrusion head of FIG.1;

c) FIG. 3 is a partial view, in longitudinal section, of a secondpreferred embodiment of the extrusion head according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows an extrusion head, indicated with referencesign 1, for depositing a coating layer of polymeric material at aradially outer position with respect of a semi-finished product (notshown in the figure) advancing within said extrusion head as indicatedby the arrow A of FIG. 3.

It is clear that the man skilled in the art can apply the extrusion head1 to cables or to articles of a different type by making, if necessary,any suitable modifications.

In the embodiment shown in FIG. 1, the extrusion head 1 comprises a headbody 2 having a longitudinal axis L-L, a conveyor element 3, a male die4 and a female die 5. The male die 4 is fitted upon the conveyor element3, whereas the female die 5 is fitted upon the head body 2. Said diesare fixed by means of a suitable fixing plate 6.

The head body 2 is provided with an inner cavity 8 coaxially extendingwith the longitudinal axis L-L and with an inlet duct 9 of the polymericmaterial, said inlet duct being formed by means of a feeding duct 10formed in the head body 2 and extending, in the illustrated embodiment,in a direction substantially perpendicular to the axis L-L.

The conveyor element 3, which is mounted in the inner cavity 8 of thehead body 2 about the longitudinal axis L-L, comprises a tubular body 11and a pair of distribution channels 12, only one of which is illustratedby a dashed line in FIG. 1.

The tubular body 11 is provided, similarly to the head body 2, with aninner cavity 13 coaxially extending with the longitudinal axis L-L andintended for receiving the semi-finished product being produced and, asstated above, advancing according to the arrow A.

The extrusion head 1 further includes a lamination channel 14, coaxiallydefined, downstream of each distribution channel 12, between a radiallyinner surface 15 of the head body 2 and a radially outer surface 16 ofthe conveyor element 3.

The end portion of said lamination channel is defined between the maledie and the female die mentioned above.

The lamination channel 14 is intended for conveying the polymericmaterial constituting the desired coating layer onto the outer surfaceof the semi-finished product being produced. To this purpose, thelamination channel 14 defines a substantially annular and continuousthrough port which is coaxial with the longitudinal axis L-L of thetubular body 11.

The distribution channels 12 are formed on the outer surface 16 of thetubular body 11 and each channel comprises a first end in fluidcommunication with the feeding duct 10. In the embodiment illustrated inFIG. 1, the distribution channels 12 have a development of thecurvilinear type, substantially of helical type, and extend on radiallyopposite sides with respect to the axis L-L.

Each distribution channel 12 defines an upstream portion U and adownstream portion D of the tubular body 11.

In particular, each distribution channel 12 carries out the function ofdistributing the polymeric material entering the extrusion head 1through the inlet duct 9 as much homogeneously as possible in thedownstream portion D of the conveyor element 3.

In such way, said material is uniformly distributed in the laminationchannel 14 and allows an optimal production of the desired coatinglayer.

In a way known per se, for example by means of suitable ducts, notshown, the inlet duct 9 (and, by this, the feeding duct 10, thedistribution channels 12 and the lamination channel 14) is incommunication with one or more extrusion screws, which are notillustrated since they are conventional per se.

In accordance with said first embodiment of the invention, thedistribution channels 12 are provided with an undercut 17 axiallyextending in the upstream portion U of the tubular body 11 along adirection substantially parallel to the longitudinal axis L-L.

In accordance with the embodiment shown in FIG. 1, the undercut 17,which is substantially U-shaped to minimize undesired phenomena ofstagnation of material, has a length of about 6 mm. Said length issuitable for giving an effective seal between the head body 2 and theconveyor element 3 in a pressure range comprised between 0 bar and 700bar.

As shown in greater detail on an enlarged scale in FIG. 2, the undercut17 comprises a bottom wall 18 which is substantially perpendicular tothe longitudinal axis L-L.

In the embodiment shown in FIGS. 1 and 2, the undercut 17 is defined inproximity of the bottom wall 19 of the distribution channel 12.

The bottom wall 18 of the undercut 17 and the bottom wall 19 of thedistribution channel 12 are connected by a curvilinear part 20 having abending radius equal to about 1.5 mm in proximity of the feeding duct.

As illustrated in FIG. 2, the undercut 17 defines an elasticallydeformable substantially lip-shaped sealing element 21 which is definedin the upstream portion U of the tubular body 11.

The bottom wall 18 of the undercut 17 is connected to said lip 21 bymeans of a curvilinear part 22 having a bending radius equal to about1.5 mm in proximity of the feeding duct.

In accordance with the described embodiment, the outer diameter of theconveyor element was equal to 250.07 mm and the inner diameter of thehead body was equal to 250.00 mm. In said case, therefore, theinterference imparted between said head body and said conveyor elementwas equal to 0.07 mm.

With reference to the preferred embodiment of the extrusion headdescribed above and illustrated in FIG. 1, the method according to theinvention for depositing by extrusion a polymeric material by means ofan extrusion head comprises, in a preferred embodiment thereof, thefollowing steps.

After having conveyed the semi-finished product produced up to thispoint, for example a cable core, obtained in a way known per se, withthe longitudinal cavity 13 of the extrusion head 1, in a first step thepolymeric material is fed within the feeding duct 10 through the inletduct 9.

In greater detail, at the same time as said semi-finished productadvances (according to the arrow A of FIG. 3), the polymeric compositionused for forming the desired coating layer is driven by one or moreextrusion screws (known per se and not shown) and is fed through theinlet duct 9 towards the lamination channel 14 thanks to thedistribution channels 12.

In a second step of the method of the invention, a part of the polymericmaterial is conveyed towards the undercut 17 formed in each distributionchannel 12.

In said step of the method of the invention, a sealing force is exertedbetween the head body 2 and the conveyor element 3 by means of thepushing action exerted by the polymeric material in the undercut 17 in adirection substantially perpendicular to the advancing direction A ofthe semi-finished product within the extrusion head.

In the embodiment illustrated in FIGS. 1 and 2, in which the undercut 17is formed in the distribution channels 12 of the conveyor element 3, thesealing action is carried out by the polymeric material which exerts apushing action of the conveyor element 3 against the head body 2, saidpushing action being exerted by the elastically deformable lip 21against the head body 2.

Following the aforementioned steps of the method, the polymericmaterial, once distributed by the distribution channels 12 on theradially outer surface 16 of the conveyor element 3 and in thelamination channel 14, is conveyed in the zone of the dies on the outersurface of the semi-finished product to produce the desired coatinglayer.

In FIG. 3 a further embodiment of the extrusion head according to theinvention is shown.

The elements of the extrusion head which are structurally orfunctionally equivalent to those previously illustrated with referenceto FIGS. 1 and 2 shall be indicated with the same reference signs andshall not be further described.

In accordance with said embodiment, an undercut 17′ is formed in thehead body 2, and precisely in the distribution channel 12 formed in thehead body 2 (and no longer in the conveyor element 3 as described withreference to the embodiment of FIG. 1).

As illustrated in FIG. 3, the distribution channel 12 defines anupstream portion U′ and a downstream portion D′ of the head body 2 andthe undercut 17′ axially extends in the upstream portion U′ of the headbody 2.

The undercut 17′ has a length of about 6 mm and is substantiallyU-shaped so as to minimize the formation of stagnation zones of thepolymeric material.

The undercut 17′ defines an elastically deformable substantiallylip-shaped sealing element 21′ which is defined in the upstream portionU′ of the head body 2.

With reference to the aforementioned further embodiment shown in FIG. 3,the method according to the invention comprises the same steps asindicated above with reference to FIG. 1 with the only difference thatin such a case the sealing action is carried out by the polymericmaterial which exerts a pushing action of the head body 2 against theconveyor element 3.

In accordance with further preferred embodiments of the invention, theextrusion head can comprise a second conveyor element coaxially arrangedwith and at a radially outer position with respect of the first conveyorelement. In such a way, it is advantageously possible to simultaneouslydeposit by co-extrusion a pair of coating layers.

In applications of this type, in which the extrusion head comprises aplurality of conveyor elements, the first embodiment of the extrusionhead according to the invention, in other words the embodiment in whichthe undercut is formed in the distribution channel of the conveyorelement, is particularly advantageous. In fact, the presence of anundercut made in this way in each conveyor element allows to accomplishan effective seal both between the head body and the first conveyorelement, as illustrated in the aforementioned embodiments, and betweenthe first and the second conveyor element.

As illustrated above, the present invention presents a plurality ofadvantages which can be achieved.

In particular, the present invention provides a method for depositing byextrusion a polymeric material by means of an extrusion head, which iscapable of achieving a seal against leakages of the polymeric material,while ensuring a constructive and assembly simplicity of the extrusionhead and of the main components thereof (conveyor element and headbody).

In the case in which said method and said extrusion head are applied incable production processes, said advantages in turn result in thepossibility of producing cables of quality, achieving both a reductionin the costs which can be attributed to the extrusion apparatuses usedto manufacture the same (in the case in point, the extrusion head) withrespect to the extrusion apparatuses of the prior art, and a reductionin the assembly time of said apparatuses.

1. A method for depositing by extrusion a polymeric material by means ofan extrusion head comprising a head body and at least one conveyorelement coaxially mounted between each other about a common axis, saidhead body comprising at least one feeding duct disposed through aradially inner surface of said head body and a radially outer surface ofsaid head body and extending in a direction substantially perpendicularto a longitudinal axis of an inner cavity of said head body, saidfeeding duct adapted for feeding said polymeric material in a laminationchannel defined between said head body and said conveyor element, saidmethod comprising a) feeding said polymeric material in said feedingduct; and b) conveying a part of said polymeric material towards atleast one undercut arranged in proximity of an end of said feeding duct,said at least one undercut defining an elastically deformablesealing-element formed on one of the inner surface of said head body andthe outer surface of said conveyor element, the sealing-element beinglip-shaped and extending in a direction substantially parallel to saidlongitudinal axis, said part of said polymeric material exerting asealing action between said head body and said at least one conveyorelement by pushing on the sealing-element.
 2. The method according toclaim 1, wherein said sealing action of said part of said polymericmaterial is carried out by pushing on the sealing-element in a directionsubstantially perpendicular to said common axis.
 3. The method accordingto claim 1, wherein said undercut is formed in at least one distributionchannel formed on the outer surface of said conveyor element, andwherein said sealing action is exerted by pushing said conveyor elementagainst said head body.
 4. The method according to claim 3, wherein saidsealing action is carried out by pushing an elastically deformablesealing element which is integrally formed with said conveyor elementand which extends in a direction substantially parallel to said commonaxis.
 5. The method according to claim 1, wherein said undercut isformed in at least one distribution channel formed on the inner surfaceof said head body and wherein said sealing action is carried out bypushing said head body against said conveyor element.
 6. The methodaccording to claim 5, wherein said sealing action is carried out bypushing an elastically deformable sealing element which is integrallyformed with said head body and which extends in a directionsubstantially parallel to said common axis.